Donald Trump has said his wife Melania's mysterious presence is part of her 'beauty,' as he opened up to Megyn Kelly about the enigmatic former first lady.

The former president, 77, sat down with Kelly for a SiriusXM interview aired on Thursday that saw him sharing insight into both Melania and his youngest son Barron, 17.

'I think part of the beauty is that mystery,' Trump said when asked what people misunderstand about his wife of more than 20 years, before comparing her with Hollywood legend Greta Garbo.

'She was a great actress, But she was very reclusive, never did an interview. I don't see Melania as like that, but she's introspective, and she's confident… She doesn't need to be out there [to get interviewed],' he explained.

'She has confidence. She has a lot of self confidence.'

Trump also gave an update on Barron, saying the teen is doing 'really well' and is a 'very good athlete.' 

'Barron's very tall – about six eight. And, and he's a good kid. He's a good looking kid. He's a great student, very good student…we're thinking about [college] right now,' he added.